Sintered fiber filter

ABSTRACT

Sintered fiber filters are provided that can afford high particle capture efficiency and/or low pressure drop during operation, and are useful in applications such as semiconductor processing. The shape of at least a portion of the individual fibers (e.g., metal fibers) used to make the filter have a three-dimensional aspect, which allows for a low packing density and high porosity filtration media. Certain filters have a cylindrical or tube-like shape with tapered ends of higher density. Methods of making such filters, for example, using axial pressing, are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/256,134, entitled Sintered Fiber Filter, filed Oct. 22, 2008, nowU.S. Pat. No. 8,097,071, which claims the benefit under 35 U.S.C.§119(e) of U.S. Provisional Patent Application Ser. No. 60/982,328,filed Oct. 24, 2007, and entitled Sintered Fiber Filter, bothincorporated by reference herein in their entirety.

BACKGROUND

1. Technical Field

The field relates to sintered metal filters and methods of making andusing the same to filter fluids, including in applications requiringhigh efficiency filtration and/or a low pressure drop across the filter.

2. Description of Related Art

Porous metal filters, for example, made from metal powder or metalfiber, are widely used in a variety of applications. For instance, insemiconductor manufacturing and other industrial processes, a very cleanenvironment often is required to produce sensitive products. Forexample, in the electronics industry, inline filters are often used tofilter particulate matter from fluids in order to reduce theintroduction of such particulate matter into the manufacturing processfor semiconductors, thereby reducing the contamination of electronicproducts. Fluids can comprise gases and/or liquids.

Some applications in the electronics industry use inline filters thatcan achieve a high efficiency rate of removal of 99.9999999%, determinedat a most penetrating particle size, i.e., 9 log reduction value (9LRV),at a rated flow. The test methodology for evaluating 9LRV rating isdescribed in Rubow, K. L., and Davis, C. B., “Particle PenetrationCharacteristics of Porous Metal Filter Media For High Purity GasFiltration,” Proceedings of the 37rd Annual Technical Meeting of theInstitute of Environmental Sciences, pp. 834-840 (1991); Rubow, K. L.,D. S. Prause and M. R. Eisenmann, “A Low Pressure Drop Sintered MetalFilter for Ultra-High Purity Gas Systems”, Proc. of the 43^(rd) AnnualTechnical Meeting of the Institute of Environmental Sciences, (1997);and Semiconductor Equipment and Materials International (SEMI) testmethod SEMI F38-0699 “Test Method for Efficiency Qualification ofPoint-of-Use Gas Filters,” all of which are incorporated herein byreference.

Another characteristic that can be important to the electronics industryis the pressure drop across inline filters. While pressure drop can varywith the flow rate of fluid through the filter and the pressure levelsof the fluid, lower pressure drops are generally preferred in theindustry. This is because some process fluids, such as gases producedfrom vaporization of liquid sources, have limited abilities topressurize the gas system; thus, filters with higher pressure dropscould adversely reduce (restrict) the flow of process fluids.Furthermore, in a typical high purity fluid supply system each componentcontributes to the overall pressure drop across the system. The fluidfilter is typically the most significant contributor to the total systempressure drop. Reducing pressure drop across each, or any, componentreduces the overall pressure drop across the system. This is desirableto the system operator, as it allows for beneficial operation economicsby reducing the system pressure supply requirements. For example, in asystem that has fluid supplied by a compressed gas cylinder, more of thevolume of the gas can be accessed for wafer processing by reducingpressure drop across the system.

SUMMARY

Described herein are sintered fiber filters. In certain embodiments, thefilters provide high efficiency removal of particulates and/or lowpressure drop during operation, and are useful in applications such assemiconductor processing. The shape of at least a portion of theindividual fibers (e.g., metal fibers) that make up the filter have athree-dimensional aspect, which allows for a low packing density andhigh porosity filtration media. Such low density/high porosity promotesa low pressure drop across the resultant filter formed from the media.Varying the degree of compression and/or varying the quantity of fibersduring molding of the filter allows for control of the filtrationrating, or particle capture efficiency and differential pressure of theresultant filter. In certain embodiments, the filter has a cylindricalor tube-like shape, in some instances having tapered ends of higherdensity that allow for welding, e.g., to a metal end cap and/or filterassembly. Methods of making such filters, for example, using axialpressing, are also disclosed.

One aspect provides a filter element including a sintered fiber metalmedia. The filter element has a cylindrical body. In some instances, thecylindrical body has an outer diameter that decreases from a centerportion of the element toward an end portion of the element, and thefiber media has a density that increases from a center portion of theelement toward an end portion of the element. In at least someembodiments, the density of the media at a center portion of the elementis about 15% or less. In certain embodiments, the element provides afiltration efficiency of at least 5 log reduction value (LRV) at a fluxof 33 SLM/in², in some instances at a flux of 37 SLM/in², and in someinstances at a flux of 37.9 SLM/in², measured at a most penetratingparticle size, with nitrogen flow and atmospheric conditions at the exitof the filter. In some embodiments, the filter element has a filtrationefficiency of at least 9 LRV at a flux of 6 SLM/in², or at a flux of 7SLM/in², or at a flux of 7.6 SLM/in², or at a flux of 106 SLM/in². Insome embodiments, the filter element is contained in a metal housing. Inother embodiments, the filter element is not contained in a metalhousing, but is affixed to hardware at each end. Nonlimiting examples ofsuch hardware include flanges, tubes, and mounts. In certainembodiments, the density of the media at a center portion of the elementis about 12% or less, for example, about 6% or less. The disclosedelement generally referred to herein as a “filter element” can also beused in other applications besides filtration. For example, such anelement may be used as a flow diffuser, a sparger, a dampener, a wick, ademister, a silencer, a straightener, or another related element.

In some embodiments, the cylindrical body has an inner diameter at thecenter of the filter element between about 0.1 inches and about 2.0inches, for example, between about 0.4 inches and about 0.8 inches. Insome embodiments, the cylindrical body has a wall thickness at thecenter of the filter element between about 0.1 inches and about 1.5inches. In certain embodiments, the filer element has a thickness ofabout 0.2 inches to about 0.3 inches at the center of the element, and athickness of about 0.1 inch at the ends of the element. In someembodiments, the length of the filter element is about 0.5 inches toabout 15 inches, for example, about 1 inch to about 5 inches, or about 2inches to about 3 inches.

In some embodiments, the filter element is used to filter a fluid. Afluid to be filtered is contacted with the filter element. In certainembodiments, the fluid is a gas. In certain embodiments, the filterprovides a pressure drop between about 2 psi and about 10 psi at a fluxof 6.8 to 42 SLM/in², with nitrogen flow and atmospheric exitconditions. In some embodiments, the filter element provides a pressuredrop between about 0.1 psi and about 5 psi at a flux of 0.8 to 42SLM/in². In other embodiments, the filter element provides a pressuredrop between about 5 psi and about 25 psi at a flux of 15 to 106SLM/in². In yet other embodiments, the filter element provides apressure drop between about 0.1 psi and about 0.5 psi at a flux of 1.6to 8 SLM/in². In certain embodiments, the filter element provides anefficiency (LRV) per unit pressure drop between about 1 and about 11psid⁻¹, at a flux of 7 to 37 SLM/in², or in another embodiment LRV perunit pressure drop could be as low as 0.4 psid⁻¹ at a flux of up to 106SLM/in².

Another aspect provides a method of making a sintered metal fiber filterelement. The method includes providing a mold having a cylindricalcavity with an end closure at one end of the cylindrical cavity, and afill cap at another end of the cylindrical cavity. The fill cap isremovable to provide an open end, and a core rod is movably sealed inthe end closure and extends coaxially within the cavity. The mold isoriented vertically with the open end disposed upwardly, and metal fiberand liquid is introduced into the cavity through the open endsubstantially radially evenly about the core rod. A pressuredifferential is created in the mold to expel liquid from the mold.Pressure is applied to the mold and thereby to the metal fiber in thecavity, so that the metal fiber coheres to form a substantiallytube-shaped structure. The substantially tube-shaped structure isremoved from the mold and sintered to obtain a porous tube-shapedsintered metal filter element. In some embodiments, the mold isvibrated. In some embodiments, the pressure differential is created witha vacuum. In other embodiments, the pressure differential is created byapplying pressure to the fill cap. In some embodiments, the ends of theporous tube-shaped sintered metal filter element are densified, forexample, by rotating the filter element while applying a rollerburnisher tool to the ends of the filter element. In another aspect, amethod of filtering a fluid is provided, where the fluid is filteredwith a filter element comprised of a sintered fiber media, where thefilter element has a cylindrical body with an outer diameter thatdecreases from a center portion toward an end portion, where the densityof the fiber media increases from a center portion to and end portion,where the density of the media at a center portion is about 15% or less,and where the filter element provides a filtration efficiency of atleast 5 LRV at a flux of 37.9 SLM/in² at a most penetrating particlesize under nitrogen flow and atmospheric conditions at filter exit.

In some embodiments, the end closure of the mold is removable. Incertain embodiments, a vacuum line is attached to the mold, and openedwhile introducing metal fiber and liquid to the cavity. In someembodiments, the tube-shaped structure is dried before sintering. Incertain embodiments, an end of the porous tube-shaped sintered metalfilter element is welded to an end cap and/or a filter housing.

BRIEF DESCRIPTION OF THE DRAWINGS

The following figures are presented for the purpose of illustrationonly, and are not intended to be limiting.

FIG. 1 is a photomicrograph of metal fiber media, after sintering, usedin certain embodiments, at a magnification of 2000 times.

FIG. 2 is a photomicrograph of metal fiber media, after sintering, usedin certain embodiments, at a magnification of 5000 times.

FIG. 3 is a photomicrograph of metal fiber media according to certainembodiments, after it has been sintered and used for filtration, at amagnification of 3500 times, laden with particulate matter that wasfiltered.

FIG. 4 is a photomicrograph of metal fiber media according to certainembodiments, after it has been sintered and used for filtration, at amagnification of 7500 times, laden with particulate matter that wasfiltered.

FIGS. 5A and 5B illustrate a forming fixture for use in fabricating afilter according to certain embodiments.

FIG. 6 is a side view of a filter assembly according to certainembodiments. The ends are densified, with a smaller outer diameter atthe ends of the filter element than at the center section. The right endof the filter element has been welded to an end cap and the left end ofthe filter element has been welded to a housing outlet.

FIG. 7 is a side view of a filter according to certain embodiments.

FIG. 8A is a top plan view of a filter according to certain embodiments.FIG. 8B is a longitudinal cross-sectional view of a filter showing thefilter element inside according to certain embodiments.

FIG. 9A is a top plan view of a filter element according to certainembodiments. FIG. 9B is a longitudinal cross-sectional view of a filterelement according to certain embodiments.

FIG. 10 is a plot of the differential pressure across a filter accordingto certain embodiments versus the flow rate through the filter atvarying pressures.

FIG. 11 is a plot of the efficiency rate of removal of a mostpenetrating particle size versus the flow rate through a filteraccording to certain embodiments.

DETAILED DESCRIPTION

Sintered fiber filters are provided that, in at least some embodiments,provide high efficiency and/or low pressure drop during operation, e.g.,for fluid filtration in semiconductor processing. In certainembodiments, the filter has a cylindrical or tube-like shape.

Filter elements as described herein are made from metal, metal oxide, orceramic material. In at least some embodiments, the filter element ismade from a metal fiber media wherein at least a portion of theindividual metal fibers that make up the media have a shape with somethree-dimensionality, which allows for a low packing density and highporosity filtration media. For example, when poured, the fibers can havea packing density as low as about 2-3%. The term “three-dimensionalaspect” or “three-dimensionality” as used herein with respect to theshape of a metal fiber refers to random directional changes in the majoraxis of the fiber compared to a theoretical straight fiber, e.g.,leading to a curved, kinked, entangled, cork screw, lazy curve, z-shape,90 degree bend, or pigtail shape. When the fibers having a shape withsome three-dimensionality are laid down or poured, they tend tointerlock, resulting in a media having a fluffy texture, with asubstantial amount of open space between the individual fibers. Incertain embodiments, at least about 5%, at least about 10%, at leastabout 20%, at least about 30%, at least about 40%, at least about 50%,at least about 60%, at least about 75%, or at least about 90% of theindividual metal fibers have a shape with a three-dimensional aspect.The percentage of fibers in the media having a shape with somethree-dimensionality is determined, for example, by examining arepresentative number of fibers under a microscope.

In some embodiments, the fibers are short metal fibers including curvedand entangled fibers. Such fibers are commercially available (e.g., fromN.V. Bekaert S.A., Belgium). An example of such fibers, and methods fortheir preparation are described in U.S. Pat. No. 7,045,219 (Losfeld etal.), which is incorporated herein by reference. As a brief summary,U.S. Pat. No. 7,045,219 discloses a set of short metal fibers including“entangled” fibers and “curved” fibers, e.g., having an equivalentdiameter between 1 and 150 microns. The entangled fibers may represent 5to 35% of the fibers, and have an average length at least 5 times theaverage length of the curved fibers. The curved fibers may have anaverage length between 10 and 2000 microns, and a portion of the curvedfibers may have a major axis that changes over an angle of at least 90degrees. The length/diameter ratio of the entire set of fibers may bemore than 5. The entangled fibers are entangled within themselves orwith each other, and the major axis of each entangled fiber changesoften and unpredictably. Some of the fibers have a chaotic shape, looklike a pigtail, or are present in a shape that resembles a clew. Whenpoured, the fibers may have an apparent density in the range of 10 to40%. The short metal fibers can be obtained by individualizing metalfibers in a carding operation, cutting or entangling and sieving thefibers, using a comminuting machine.

As a result of their shapes, the fibers employed according to variousembodiments herein tend to have a low packing density. Thus, for a givenvolume of fibers, a significant portion of the volume is empty orambient space, i.e., the porosity tends to be high. This low packingdensity/high porosity allows the filters made from such fibers toexhibit a low pressure drop as fluid flows through the filter. The lowpacking density of the fibers can be seen in FIGS. 1 and 2, whichillustrate fibers used in certain embodiments, after they have beensintered, under high magnification. FIG. 1 shows the fibers at 2000times magnification and FIG. 2 shows the fibers at 5000 timesmagnification. FIGS. 3 and 4 show the metal fiber media according tocertain embodiments, after it has been sintered and used for filtration,laden with particulate matter that was filtered. FIG. 3 shows amagnification of 3500 times, and FIG. 4 shows a magnification of 7500times.

Useful materials for making the fibers of some embodiments include, butare not limited to, one or more of stainless steel, including 316Lstainless steel, nickel, thallium, titanium, aluminum, tungsten, copper,metal oxides, ceramic materials, and alloys, such as Hastelloys, bronze,Cu-alloys, and Fe—Cr—Al alloys.

Exemplary dimensions for the fibers used according to variousembodiments include fiber equivalent diameters of about 1 micron toabout 150 microns, for example, about 1 micron to about 75 microns,about 1 micron to about 50 microns, about 1 micron to about 35 microns,or about 1 micron to about 10 microns; and fiber lengths of about 10microns to about 2000 microns, for example, about 10 microns to about1000 microns, about 10 microns to about 200 microns, or about 10 micronsto about 100 microns. The “equivalent diameter” of a fiber refers to thediameter of a circle having the same cross-sectional area as the fibercut perpendicular to its major axis. The length of a fiber refers to thedistance along its major axis if the fiber were straightened out suchthat there is no change in the major axis of the fiber.

In certain embodiments, a method of making a filter or filter media fromsuch fibers is disclosed. A non-limiting example of such a methodincludes molding the fiber metal media into the desired shape, e.g., acylindrical or tube-like filter. In at least some instances, the moldingis performed by axial pressing. The molding can also be performed byother pressing methods, e.g., isostatic pressing. In certainembodiments, a fiber material is measured and mixed with a liquid toform a mixture that is molded using a forming fixture. Nonlimitingexamples of liquids with which a fiber material may be mixed includewater, water-based solutions, alcohol, alcohol-based solutions,glycerin, and mixtures thereof. In some embodiments, the mixture is freeof binders. Alternatively, the fiber can be molded dry by, for example,air classification. Various methods of compaction to achieve a desireddensity are well known.

A non-limiting example of a suitable forming fixture for making a filterelement as described herein includes a cylindrical assembly for axialpressing. In one such assembly, illustrated in FIGS. 5A and 5B, aforming fixture 500 includes a forming tube 501, two end caps 502, 503,one larger porous washer 504, two smaller porous washers 505, and twopush tubes 506. For example, the forming tube 501 is a metal cylinder,e.g., stainless steel with a hollow core. In the illustrated embodiment,near the bottom end of the forming tube 501 is a notch 507 in the outersurface of the tube fitted with a gasket 508, e.g., an o-ring. As anon-limiting example, in the illustrated embodiment, the forming tube501 is 11 inches long, with an outer diameter of approximately 1.6inches, and an inner diameter of approximately 1.2 inches. Thedimensions of the forming tools are adjusted such that fiber tubes ofdifferent dimensions and densities can be produced.

The forming tube 501 is mated with the two end caps 502, 503, e.g., madeof plastic, one for each end. In certain embodiments, the bottom end cap502, which can mate with the bottom end of the forming tube 501, has avalve 509 to which a vacuum line can be attached. In at least some suchembodiments, the top end cap 503 is a fill cap that has a rod hole 510through the middle of the cap through which the core rod 511 can extend,additional flow holes 512 around the rod hole 510 through which thefiber and liquid mixture can flow, and a reservoir top 513 into whichthe fiber and liquid mixture can be poured before flowing through theflow holes 512. Both end caps 502, 503 are designed to fit over theforming tube 501. As such, both end caps 502, 503 have an inner hollowedout section with a diameter substantially the same as an outer diameterof the forming tube 501.

The core rod 511, e.g., made of stainless steel, is used within theforming tube 501. In certain embodiments, the core rod 511 is smaller indiameter (e.g., in the illustrated embodiment, approximately 0.5 inchesin diameter) and slightly longer than the inside of the forming tube501, such that when the bottom end of the forming tube 501 is mated withthe bottom end cap 502, the forming tube 501 is placed around the corerod 511 and inserted into the bottom end cap 502, and the fill cap 503is attached to the top end of the forming tube 501, the top end of thecore rod 511 is substantially flush with a top of the rod hole 510 inthe fill cap 503.

The smaller porous washers 505 have an inner diameter substantiallyequal to the diameter of the core rod 511 and an outer diametersubstantially equal to an inner diameter of the forming tube 501. Thelarger porous washer 504 also has an inner diameter substantially equalto the diameter of the core rod 511, but has an outer diametersubstantially equal to the outer diameter of the forming tube 501. Thetwo push tubes 506 have an outer diameter smaller than the innerdiameter of the forming tube 501 and an inner diameter larger than thediameter of the core rod 511. In some embodiments, the push tubes 506are stainless steel. As a non-limiting example, in the illustratedembodiment, the push tubes 506 are each approximately 7 inches long. Thedimensions of the forming tools are adjusted such that fiber tubes ofdifferent dimensions and densities can be produced.

To assemble the forming fixture 500, the bottom end cap 502 is placed ona surface, and the larger porous washer 504 is placed inside the end cap502. The bottom end of the forming tube 501 is then placed into thebottom end cap 502 such that the gasket 508 creates a seal with the endcap 502. The forming tube 501 is pressed down into the bottom end cap502 until the forming tube bottoms out on the larger washer 504 in thebottom end cap 502. The core rod 511 is then placed through the formingtube 501 and through the larger washer 504 such that the core rod 511bottoms out in the bottom end cap 502. A smaller washer 505 is thenplaced on the core rod 511 and inserted into the forming tube 501. Thesmaller washer 505 slides all the way down the core rod 511 so that itrests on top of the larger washer 504. The fill cap 503 is then placedonto the core rod 511 and onto the top end of the forming tube 501. Thefill cap 503 fits snugly onto the top end of the forming tube 501 toavoid leakage when the fiber and liquid mixture is poured into the fillcap 503.

In one non-limiting example of a method for making a filter as describedherein, the assembled forming fixture 500 is placed on a vibratingtable, for example, a No. 200 Extra-Heavy Duty Vibrator (Buffalo DentalManufacturing Co., Syosset, N.Y.). A vacuum line is then attached to thevalve 509 on the bottom end cap 502, and the vacuum is turned on, in atleast some instances initially to a low vacuum setting. The vibratingtable is then turned on. The fiber and liquid mixture is well-mixed, anda small amount is poured into the fill cap 503, after which the vacuumline is fully opened. The fiber and liquid mixture typically iscontinually mixed as it is poured into the fill cap 503 and flows intothe space around the core rod 511 and within the inner wall of theforming tube 501. After all the fiber and liquid mixture has been pouredinto the forming tube 501 through the fill cap 503, additional liquid ispoured into the fill cap 503 to clean any leftover fiber material thatdid not flow through the fill cap 503 and into the forming tube 501. Insome alternative embodiments, a vibrating table is not employed. In somealternative embodiments, instead of employing a vacuum, a pressuredifferential is created to remove liquid from the mold by applyingpressurized gas to the fill cap end of the mold.

Once the liquid level has dropped into the forming tube 501, the fillcap 503 is removed from the forming tube 501. A second smaller porouswasher 505 is then placed onto the core rod 511 and into the formingtube 501. A first push tube 506 is then placed around the core rod 511,inside of the forming tube 501, and on top of the second smaller washer505. The gravitational force of the first push tube 506 slowly pushesthe second smaller washer 505 down into the tube. Once the first pushtube 506 stops moving down the forming tube 501, the vibrating table isturned off and the vacuum is turned off. The vacuum is bled off and thevacuum line is then removed from the valve 509 on the bottom end cap502.

The forming tube 501 is then placed on its side, and the bottom end cap502 and the larger washer 504 are removed from the forming tube 501. Thesecond push tube 506 is inserted into the forming tube 501 through thebottom end of the forming tube 501 so it is adjacent a smaller washer505. The second tube is then pushed into the forming tube 501, e.g.,approximately three inches. The forming fixture 500, including theforming tube 501, the core rod 511, and the two push tubes 506, is thenpicked up and stood on any hard surface with the bottom end down.Downward pressure is applied to the top push tube 506, furthercompressing the forming fixture 500. By way of non-limiting example, inthe illustrated embodiment, the forming fixture is compressed until thetotal length of the fixture 500, including both push tubes 506 extendingfrom the forming tube 501, is approximately 17 inches, yielding a greenfiber tube approximately 3.2 inches long. The dimensions of the formingtools are adjusted such that fiber tubes of different dimensions anddensities can be produced.

The forming fixture 500 is then stood up on a table with the bottom endup and the top end down on the table. The push tube 506 that is now onthe top of the forming tube 501 is removed, and the forming tube 501 ispushed down until it bottoms out on the table top. The push tube 506that was removed is then put under the push tube 506 that is now on thebottom of the forming tube 501, so that the push tubes 506 are adjacent.When they are adjacent, the forming tube 501 is pushed down to the tablewhile the core rod 511 is held steady. This pushes the formed fiber tubeout of the forming tube 501. The two smaller washers 505 are thenremoved from the core rod 511, and the formed fiber tube is also removedfrom the core rod 511.

The formed fiber tube is then dried and sintered to form a filterelement. In certain embodiments, the formed fiber tube is placed on asintering tray, between two fixed rings. The fixed rings prevent theformed fiber tube from expanding as it dries. The formed fiber tube isdried at a temperature of about 70 to about 200 degrees C., for example,about 75 degrees C., in an oven, for between about 2 hours and about 12hours, for example, about 3.5 hours. The dried formed fiber tube is thensintered in a furnace, for example in a vacuum furnace or in atmosphericconditions with nitrogen or hydrogen on a conveyor belt, for about 30 toabout 120 minutes, for example, about 60 minutes, at about 1800 to about2100 degrees F., for example, about 1900 degrees F.

After cooling, the sintered fiber tube is capable of filtration. Incertain embodiments, however, the sintered fiber tube is welded into afilter housing or other hardware. In some such embodiments, the ends ofthe fiber tube are densified to facilitate welding, which can bedifficult for a fiber tube having a low packing density. In certainembodiments, a sintered fiber tube is densified at the ends whileretaining its filtering characteristics by contouring both ends of thetube so that the outer diameter gradually decreases near the ends. As anon-limiting example, in some embodiments this is achieved by spinningthe sintered filter tube on a lathe and gradually compressing thesintered filter tube at the ends by contacting the spinning sinteredfilter tube with an external wheel. In at least some instances, this isaccomplished by burnishing with a roller burnisher tool. Densifyingcompresses the ends of the tube, for example, by a factor of about 1.5to about 3.5, and in some instances about 2.5. For example, in someembodiments, the sintered filter tubes have a wall thickness along theirentire lengths, before being densified, of approximately 0.25 inches toapproximately 0.30 inches. In some such embodiments, after beingdensified, the sintered filter tubes have a wall thickness at their endsin the range of approximately 0.06 inches to approximately 0.10 inches.In some alternative embodiments, the tube is not densified, e.g., havingapproximately uniform density and wall thickness throughout.

An axial pressing operation such as that described with respect to FIG.5 has been surprisingly found to provide advantages. However, othertypical methods of pressing fiber metal, such as isostatic pressing, orisopressing, are also contemplated. Isopressing can be done, forexample, by placing the fiber mixture in a pressure vessel, in which itsis compacted radially inward.

It was not expected that axial pressing, e.g., pressing along the axisof a cylinder, as in some embodiments described herein, would producesuitable fiber tubes. That is because axial pressing traditionallyinvolves an uneven force distribution that produces a density gradientover the length of a pressed cylinder, so that the density decreasestoward the axial center along its length. However, it was found thataxial pressing methods as described herein, for example, as illustratedand described with respect to FIG. 5, reduce this gradient to levelsthat do not substantially interfere with filter performance. While notto be bound by theory, it is believed that a density gradient is formedover the length of a cylindrical fiber part when a fiber and liquidmixture is poured into a mold; axial pressing tends to reduce thisgradient. This reduction, in part, occurs since the presence of theliquid assists in providing a more uniform distribution of thecompacting force (and thus pressure) throughout the fiber tubularelement and aids lubricity to reduce fiber drag on the tooling walls.However, dry pressing of the fibers is also contemplated. Axial pressingalso advantageously provides substantially uniform inner and outerdiameters for the filter tube. As one skilled in the art willappreciate, density gradients can be controlled to a certain extent bycontrolling parameters such as the wall thickness and length of thefilter tube, levels of vibration, fill rate and homogeneity of the fiberand liquid mixture, and rate of compaction.

In certain embodiments, once densified, the sintered fiber tubes arewelded at the densified ends. For example, to one end of the tube iswelded a metal end cap, which precludes the flow of fluid through thatend of the tube, and to the other end of the tube is welded a housingoutlet through which fluid can flow. FIG. 6 shows a densified sinteredfilter tube 600 according to certain embodiments, to which has beenwelded an end cap 602 on the right side and a housing attachment 604 onthe left side. Typically, densification at the ends of a filter elementto facilitate welding can lead to the creation of crevices in thesintered fiber tube, or other damage to the structure. However, in atleast some embodiments, for example, as illustrated in FIG. 6, theseproblems are reduced by contouring the densification, such that thedensity increases gradually from a center portion of the sintered filtertube 608 to an end portion of the tube 606. The tube 600 is tapered orcontoured from the center portion 608 to the end 606, reflecting thisdensity gradient. Creating a density gradient that gradually increasestoward the end of the filter tube helps to prevent imperfections in thefiber structure that might otherwise result from abrupt changes indensity or the densification process. The density gradient is createdduring the densification process of the sintered filter element, forexample, by burnishing with a roller-burnisher tool as described herein.

FIG. 7 shows a non-limiting example of a filter housing 700 that hasbeen constructed around a filter such as the one shown in FIG. 6,creating a finished inline porous metal filter. The housing 700 istypically metal, such as stainless steel, e.g., 316L stainless steel. Incertain embodiments, such a housing in combination with a filter tube asdescribed herein provides high temperature tolerance, for example, insome embodiments up to about 450 degrees C. In various embodiments,alternative housings are employed, including standard filter housingsused in the field.

FIGS. 8A-B illustrate a filter tube 600 according to certainembodiments, with end cap 602 and filter housing attachment 604,contained within filter housing 700. The housing attachment 604 providesa non-limiting example of a part suitable for attaching the filter tube600 to the housing 700. The adapter 604 includes an interior ringportion 802 for securing the end of the filter element 600, and a screwportion 804 for inline attachment to a fluid feed during operation.

Referring to FIG. 8B, the filter is placed inline in a fluid flow. Fluidcan flow into and through the filter housing from either end duringoperation. For example, in some instances fluid flows in from the rightside of the figure, through the right opening 701 of the housing 700.Fluid cannot penetrate the end cap 602 on the right side of the filtermember 600, so it flows around the end cap 602 and around the filterelement 600. The pressure of the flow forces the fluid through thefilter element 600, thus filtering a very large proportion of theparticulate matter in the fluid. The fluid penetrates the filter element600, is filtered by the filter element 600, and flows out the filterelement 600 and out of the housing 700 through the opening 702 on theleft side of the figure.

FIGS. 9A-B illustrate a cylindrical filter element 600 according tocertain embodiments. In some instances, the inner diameter of the filter600 is between about 0.1 inches and about 2 inches, for example, about0.3 inches to about 1 inch, about 0.4 inches to about 0.8 inches, orabout 0.6 inches. In some instances, the wall thickness in the center ofthe filter element 600 is between about 0.1 inches and about 1.5 inches,for example, about 0.25 inches to about 1 inch, or about 0.5 inches. Asillustrated in FIG. 9, in certain embodiments, the outer diameter tapersfrom a center portion 608 of the filter element 600 toward an endportion 606 of the filter element 600, reflecting densification of theends 606 of the filter element 600. For example, in certain embodiments,the cylinder has a thickness of about 0.2 inches to about 0.3 inches inthe center of the filtration element, and a thickness of about 0.1 inchat the ends of the filtration element. In some instances, the length ofthe cylindrical filter 600 is about 0.5 inches to about 15 inches, about1 inch to about 4 inches, or about 2 inches to about 3 inches, forexample, about 2.8 inches.

Because of the three-dimensional shape or curvature of the fibersemployed, filter elements as described herein exhibit low density/highporosity in at least some embodiments. In at least some instances thedensity of a sintered fiber filter element ranges from about 2% to about30%, for example, from about 5% to about 18%, about 5% to about 13%,about 4% to about 8%, or about 10% to about 14%. In some instances, thedensity of a filter is about 6%, about 12%, or about 17% to about 18%.The density is adjustable, for example, based on the degree ofcompression and/or the quantity of fibers employed in manufacturing thefilter element. Higher densities are achieved using greater compressionand/or larger quantities of fiber. In some instances, a higher densityis employed to promote higher capture efficiency. In some instances, alower density is employed to promote lower pressure drop across thefilter.

In part due to the low densities, in at least some embodiments, filtersas described herein provide low pressure drops during operation. As anon-limiting example, in high-pressure compressed gas lines, where thepressure of the gas being filtered can exceed 1000 psi, some filters asdescribed herein afford pressure drops ranging from about 2 psi to about10 psi. For applications in lower pressure environments, filtersaccording to some embodiments afford pressure drops ranging from about0.1 psi to about 5 psi, for example, from about 0.2 psi to about 1 psi.

FIG. 10 is a plot of the differential pressure across a filter versusthe flow rate through the filter at varying pressures. The pressure dropdata in FIG. 10 represent 4 sets of data for a filter made according toExamples 1 and 2 below. The fluid in all cases was high pressureultra-high purity nitrogen. The gas flow rate was measured with a massflow meter which was located downstream of the filter and thebackpressure control valve (when used). For one set, the pressure at theexit of the filter is at atmospheric conditions (nominally 1 atm and 70°F.) and the upstream pressure increased to obtain the associated flowrate and differential pressure across the filter. For the other 3 cases,the inlet pressure was held constant at either 30, 60 or 90 psig and thepressure drop across the filter monitored while a valve locateddownstream of the filter was used to control both gas flow rate and backpressure at the exit of the filter.

The filters in at least some embodiments provide high captureefficiency. In some embodiments, efficiencies of 99.9999999% or greater,determined at a most penetrating particle size, i.e., 9 log reductionvalue (9LRV), are provided. In other embodiments, lower efficiencies,such as 5LRV or greater, are employed, for example, if very highefficiency is not required, or particularly low pressure drop isdesired. FIG. 11 is a plot showing the relationship between filterefficiency and flow rate through a filter made as described in Examples1 and 2, measured at a most penetrating particle size. The theoreticalcurve was obtained using particle collection theory for fibrous media asdeveloped by Rubow, explained in Rubow, K. L., “Submicron AerosolFiltration Characteristics of Membrane Filters”, Ph.D. Thesis,University of Minnesota, Mechanical Engineering Department, Minneapolis,Minn. (1981).

The low packing densities of the filters of some embodiments allow for alower pressure drop per unit level of particle capture. Put another way,the filters of some embodiments allow for a high level of particulatecapture per unit of pressure drop. While this comparison can be made atany particle size, using the particle capture efficiency as measured ata most penetrating size represents the most difficult particle size tocapture, i.e., the particle size with the lowest capture efficiency orlowest LRV. Unless expressly indicated otherwise, all LRV valuesidentified herein are measured at a most penetrating particle size.Furthermore, the ratio of LRV to pressure drop at a given flow rate canalso be computed at any system pressure, e.g., as illustrated for the 4different pressure drop curves presented in FIG. 10. For consistency,this ratio is computed for the case where pressure at the exit of thefilter is at atmospheric conditions (nominally 1 atm and 70° F.), thegas is ultra-high purity nitrogen, and the upstream pressure increasedto obtain the associated flow rate and differential pressure across thefilter, as presented in FIG. 10. This ratio is also dependent on gasflux rate (velocity), which is flow rate per unit effective filtersurface area. For a cylinder, the effective area is computed based onthe diameter at the midpoint of the wall thickness (calculated for thetube prior to densifying the ends). In the case of a filter preparedaccording to Examples 1 and 2, the effective diameter is 0.75 in andresultant effective area is 6.6 inch. At flow rates of 50, 75 and 250SLM, the resultant flux is 7.6, 11.4 and 37.9 SLM/inch². The ratio ofLRV to pressure drop at these 3 flow rates is 10.1, 5.7 and 1.0 psid⁻¹,respectively.

Particle retention testing was performed using the following procedure.Each filter was challenged at its maximum rated flow with polydispersedNaCl particles. The mean size of the particles was 0.07 μm, which is inthe vicinity of the most penetrating particle size. The test filter waspurged with compressed filtered ultra-high purity nitrogen gas atambient temperatures. The particle background counts were maintained atzero prior to initialization of the particle challenge portion of thetest. The particle concentration upstream and downstream of the testfilter was simultaneously measured with two condensation particlecounters (CPC). The particle retention results are listed as logreduction value (LRV). LRV is the log of the ratio of particleconcentration upstream of the filter to particle concentrationdownstream of the filter. The test methodology for evaluating high LRVrating is described in Rubow et al.,“A Low Pressure Drop Sintered MetalFilter for Ultra-High Purity Gas Systems,” Proceedings of the 43rdAnnual Technical Meeting of the Institute of Environmental Sciences, pp.834-840 (1991), and Rubow, K. L., D. S. Prause and M. R. Eisenmann, “ALow Pressure Drop Sintered Metal Filter for Ultra-High Purity GasSystems”, Proc. of the 43^(rd) Annual Technical Meeting of the Instituteof Environmental Sciences, (1997), which are incorporated herein byreference.

For example, a cylindrical filter prepared according to Examples 1 and 2was measured to provide a removal rating at a most penetrating particlesize, with efficiency exceeding 9 LRV at a flow of less than 50 SLM, andexceeding 5 LRV at a flow of less than 250 SLM.

In some alternative embodiments, processes similar to those describedabove are used to produce filters having different shapes, such as astar or pleated shape. These shapes can have internal features thatcorrespond to the contour of the external feature, are cylindricaland/or some combination of these. These varying shapes are createdthrough the use of a die or mold that corresponds to the resultantshape. In some embodiments, processes similar to those described aboveare used to produce filters with non-circular cross-sections, in aninner diameter, an outer diameter, or both. In some embodiments,resulting filters have a non-uniform shape along a length of the filter,in an inner diameter, and outer diameter, or both. By way of nonlimitingexample, processes similar to those described above are used to producefilters with a star-shaped outer surface at a center portion of thefilter, and a cylindrical shaped outer surface at the end portions ofthe filter. This can be accomplished, for example, by using a formingtube that has a star-shaped interior at a center portion and acylindrical shaped interior at the end portions. The inner surface ofthe filter can similarly have a non-uniform shape based on thecorresponding shape of the core rod.

Filters as described herein are useful in a variety of applicationswhere fiber filters are desired. For example, as will be understood bythose skilled in the art, filters according to certain embodiments areprovided in a housing or affixed to other hardware such as a flange ormount for incorporation into a system that provides gases forsemiconductor processing, e.g., in compressed gas lines, and processesused in the biopharmaceutical industry.

The following non-limiting examples further illustrate certainembodiments.

EXAMPLE 1

A cylindrical filter element was made using metal fibers as described inU.S. Pat. No. 7,045,219 (N.V. Bekaert S.A., Belgium—Bekinox SF 1.5μm/316 LV Z60). The fibers were 316L stainless steel, about 1.5 micronsin diameter and nominally about 75 to about 100 microns long. 22 g ofmetal fiber was measured into a glass beaker. 1000 mL deionized waterwas measured into a plastic Tri-Pore beaker, and 200 mL deionized waterwas measured into a separate plastic Tri-Pore beaker. The 22 g of fiberwas mixed into the 1000 mL deionized water and stirred with a glassstirring rod until thoroughly mixed. The fiber/water mixture was pouredinto and compressed using a forming fixture as illustrated in FIG. 5,using a vibrator table and vacuum, as described in detail above withreference to operation of the fixture of FIG. 5. The 200 mL additionaldeionized water was used to clean remaining fiber into the toolingbefore compression. Compression yielded a green fiber tube approximately3.2 inches long. The fiber tube was removed from the forming fixture andplaced onto a sintering tray between two rings. The tube was dried at 75degrees C. in an oven for at least 3.5 hours, and then sintered in avacuum furnace at 1900 degrees F. for 60 minutes. The resultant tube hada 1.10 inch outside diameter and a 0.41 inch inside diameter. The filterelement was then cut to a length of 2.8 inches and the ends rollerburnished to achieve the contoured shaped shown in FIG. 9 with theoutside diameter of each end at 0.80 inch.

EXAMPLE 2

The filter element made according to Example 1 was then subsequentlywelded and assembled to achieve a filter as shown in FIGS. 6 and 8. Thisfilter was subsequently tested to obtain the pressure drop data andparticle collection efficiency data shown in FIGS. 10 and 11,respectively.

A filter particle loading test was performed on a filter made accordingto Example 1. The air flow rate was 100 SLM, the particle size was 0.07microns, which was determined to represent the most penetrating particlesize, the challenge concentration was 20,000 particles per cubiccentimeter, and the initial LRV was 7.3. The total particle challengefor this test was five trillion particles, and the final LRV was greaterthan 9. The initial pressure drop was 1.8 psid and the final pressuredrop was 2.1 psid, which represented an increase of 0.3 psid or 16%.These results demonstrate that the filter pressure drop increased only amodest amount while the filter was subjected to a high degree ofparticle loading (i.e., high relative to values found in thesemiconductor industry).

EXAMPLE 3

A cylindrical filter element was made using metal fibers as described inU.S. Pat. No. 7,045,219 (N.V. Bekaert S.A., Belgium—Bekinox SF 1.5μm/316 LV Z60). The fibers were 316L stainless steel, about 1.5 micronsin diameter and nominally about 75 to about 100 microns long. 44 g ofmetal fiber was measured into a glass beaker. 1500 mL deionized waterwas measured into a plastic Tri-Pore beaker, and 200 mL deionized waterwas measured into a separate plastic Tri-Pore beaker. The 44 g of fiberwas mixed into the 1500 mL deionized water and stirred with a glassstirring rod until thoroughly mixed. The fiber/water mixture was pouredinto and compressed using a forming fixture as illustrated in FIG. 5,using vacuum, as described in detail above with reference to operationof the fixture of FIG. 5. The 200 mL additional deionized water was usedto clean remaining fiber into the tooling before compression.Compression yielded a green fiber tube approximately 3.2 inches long.

The fiber tube was removed from the forming fixture and placed onto asintering tray between two rings. The tube was dried at 75 degrees C. inan oven for at least 3.5 hours, and then sintered in a vacuum furnace at1900 degrees F. for 60 minutes. The resultant tube had a 1.10 inchoutside diameter and a 0.41 inch inside diameter. The filter element wasthen cut to a length of 2.8 inches and the ends roller burnished toachieve the contoured shaped shown in FIG. 9 with the outside diameterof each end at 0.80 inch to 1.00 inch was then subsequently welded andassembled to achieve a filter as shown in FIGS. 6 and 8.

Pressure drop and efficiency tests were performed on a 1.40 inch long,non-contoured, section of the filter element. The pressurized gas wasnitrogen and both the pressure drop and efficiency measured withatmospheric gas conditions at the filter exit. The particle size was0.07 microns, which was determined to represent the most penetratingparticle size. This filter element produces a particle retention levelof >9 LRV at a gas flow rate of 150 SLM and pressure drop of 13.1 psid.The particle retention level was also >9 LRV at a gas flow rate of 175SLM and pressure drop of 14.9 psid. The particle retention level wasalso >9 LRV at a gas flow rate of 250 SLM and pressure drop of 19.2psid. The pressure drop was 5.1, 9.2, 12.9, and 16.3 psid at flow ratesof 50, 100, 150, and 200 SLM, respectively.

Pressure drop and efficiency tests were also performed on a 1.00 inchlong, non-contoured, section of the filter element. The pressurized gaswas nitrogen and both the pressure drop and efficiency measured withatmospheric gas conditions at the filter exit. The particle size was0.07 microns, which was determined to represent the most penetratingparticle size. This filter element produces a particle retention levelof >9 LRV at a gas flow rate of 250 SLM and pressure drop of 25.0 psid.The pressure drop was 7.1, 12.4, 17.0 and 21.4 psid at flow rates of 50,100, 150, and 200 SLM, respectively.

EXAMPLE 4

A cylindrical filter element was made using metal fibers as described inU.S. Pat. No. 7,045,219 (N.V. Bekaert S.A., Belgium—Bekinox SF 1.5μm/316 LV Z60). The fibers were 316L stainless steel, about 1.5 micronsin diameter and nominally about 75 to about 100 microns long. 11 g ofmetal fiber was measured into a glass beaker. 500 mL deionized water wasmeasured into a plastic Tri-Pore beaker, and 100 mL deionized water wasmeasured into a separate plastic Tri-Pore beaker. The 11 g of fiber wasmixed into the 500 mL deionized water and stirred with a glass stirringrod until thoroughly mixed. The fiber/water mixture was poured into andcompressed using a forming fixture as illustrated in FIG. 5, usingvacuum, as described in detail above with reference to operation of thefixture of FIG. 5. The 100 mL additional deionized water was used toclean remaining fiber into the tooling before compression. Compressionyielded a green fiber tube approximately 3.2 inches long.

The fiber tube was removed from the forming fixture and placed onto asintering tray between two rings. The tube was dried at 75 degrees C. inan oven for at least 3.5 hours, and then sintered in a vacuum furnace at1900 degrees F. for 60 minutes. The resultant tube had a 0.865 inchoutside diameter and a 0.550 inch inside diameter and a density range ofbetween 6 and 7.2 percent dense. The filter element was then cut to alength of 2.8 inches and the ends roller burnished to achieve thecontoured shape shown in FIG. 9 with the outside diameter of each end ata range of 0.80 to 0.60 inch. This filter element produces a particleretention level of 6.6 LRV at a gas flow rate of 50 SLM and pressuredrop of 0.53 psid. The pressure drop was 0.09, 0.16, 0.23, and 0.33 psidat flow rates of 5, 10, 20, and 30 SLM, respectively. The test gas waspressurized nitrogen and both the pressure drop and efficiency measuredwith atmospheric gas conditions at the filter exit. The particle sizewas 0.07 microns, which was determined to represent the most penetratingparticle size.

As will be apparent to one of ordinary skill in the art from a readingof this disclosure, the present invention can be embodied in forms otherthan those specifically disclosed above. The particular embodimentsdescribed above are, therefore, to be considered as illustrative and notrestrictive. The scope of the invention is as set forth in the appendedclaims, rather than being limited to the examples contained in theforegoing description.

1-23. (canceled)
 24. A method of making a sintered fiber filter element,the method comprising: (a) providing a mold having a cylindrical cavitywith an end closure at one end of the cylindrical cavity, and a fill capat another end of the cylindrical cavity, wherein the fill cap isremovable to provide an open end, and a core rod movably sealed in saidend closure and extending coaxially within said cavity; (b) orientingsaid mold vertically with said open end disposed upwardly; (c)introducing fiber and liquid into said cavity through said open endsubstantially radially evenly about said core rod; (d) creating apressure differential in the mold to expel liquid from the mold; (e)applying pressure to said mold and thereby to said fiber in said cavity,whereby said fiber coheres to form a substantially tube-shapedstructure; (f) removing said substantially tube-shaped structure fromsaid mold; and (g) sintering said substantially tube-shaped structure toobtain a porous tube-shaped sintered filter element.
 25. The method ofclaim 24, further comprising densifying the ends of said poroustube-shaped sintered filter element.
 26. The method of claim 25, whereindensifying the ends of said porous tube-shaped sintered filter elementincludes rotating said filter element while applying a roller burnishertool to the ends of said filter element.
 27. The method of claim 25,wherein densifying the ends of said porous tube-shaped sintered filterelement includes rotating said filter element while applying a tool toat least one end of said filter element, and wherein the tool isdesigned to provide a gradual transition from a center portion of thefilter element to an end of the filter element.
 28. The method of claim24, wherein the fiber includes metal fiber.
 29. The method of claim 28,wherein the metal includes stainless steel.
 30. The method of claim 24,wherein the liquid includes water.
 31. The method of claim 24, whereinthe end closure is removable.
 32. The method of claim 24, furthercomprising attaching a vacuum line to the mold, and opening the vacuumline while introducing fiber and liquid to the cavity.
 33. The method ofclaim 24, further comprising drying said tube-shaped structure beforesintering said tube-shaped structure.
 34. The method of claim 24,further comprising welding an end of said porous tube-shaped sinteredfilter element to an end cap.
 35. The method of claim 24, furthercomprising welding an end of said porous tube-shaped sintered filterelement to a filter end fitting or housing.
 36. The method of claim 24,further comprising applying pressure to the sintered tube-shapedstructure to control the density, shape, or length of the structure. 37.The method of claim 24, further comprising vibrating the mold.
 38. Themethod of claim 24, wherein creating a pressure differential in the moldto expel liquid from the mold comprises applying a vacuum to the mold.39. The method of claim 24, wherein creating a pressure differential inthe mold to expel liquid from the mold comprises supplying pressurizedgas to the open end of the cavity.